1. Field of the Invention
The present invention relates to a base station receiving system in a mobile radio communication system. In particular, the present invention relates to a baseband signal demodulating apparatus and method in a code division multiple access (CDMA) type mobile radio communication system which can share a CDMA signal processor by a time division multiplexing method by constructing the processor for processing input multichannel signals with the minimum number of elements.
2. Description of the Related Art
Generally, in processing a transmitted/received signal in a base station transmitting/receiving system of a mobile radio communication system, its frequency band may be divided into a baseband, an intermediate frequency band, and a radio frequency band. The baseband is a frequency band occupied by the original information signal which is not modulated. Accordingly, the baseband is a low frequency band which is nearly zero or which includes a direct current (DC) component. The intermediate frequency band is a frequency band wherein the baseband frequency to be transmitted to the outside is heightened by adding thereto a local oscillating frequency, while the radio frequency received from the outside is lowered by subtracting therefrom the local oscillating frequency, by means of a frequency converter provided in the base station. The radio frequency band is a frequency band which is used for radio communications, and generally means the frequencies in the electromagnetic spectrum range of 10 KHzxcx9c300 GHz.
Generally, in the CDMA type mobile radio communication system, the respective base station transmitting/receiving system includes a CDMA baseband demodulating apparatus which demodulates a CDMA baseband signal transmitted from a mobile unit to a radio frequency (RF) signal to transmit the radio frequency signal.
Such a CDMA baseband demodulating apparatus is disclosed in U.S. Pat. No. 5,654,979 issued to Levin. According to U.S. Pat. No. 5,654,979, the conventional CDMA baseband demodulating apparatus includes an analog-to-digital (A/D) converter for converting an input analog RF signal into a digital signal and repeatedly oversampling the digital signal for a predetermined period, a plurality of rake receivers for processing multipath signals outputted through the A/D converter to produce a single signal and producing an early signal and a late signal, which are respectively earlier and later than an on-time signal, based on the on-time signal determined by selecting one among the oversampled signals that is synchronized with transmitted PN sequence most superior intensity, a plurality of correlators for obtaining correlation values of the on-time signal, early signal, and late signal outputted from the front end of rake receivers in accordance with pseudo-noise codes generated by a pseudo-noise code generator, and an accumulator for accumulating output signals of the correlators.
According to the conventional CDMA baseband demodulating apparatus as described above, baseband signal from analog receiver which converts analog RF signal into a digital baseband signal is oversampled 8 times per 1 PN sequence, called xe2x80x9cchipxe2x80x9d in the A/D convertor 90. Thereafter, to maintain the synchronization of PN sequence and the received baseband signals, the early signal and late signal, which are earlier and later than the on-time signal by the predetermined number of samples, respectively, are determined based on the on-time signal which is a signal synchronized with the pseudo-noise code.
At this time, if each front end rake receiver, which processes signals through one channel, is normally provided with 4 finger sections. The transmitted signal may be received through a direct path, while it may be received through the multipaths caused by reflection or refraction of the signal. The reason why 4 finger sections are employed is to receive multiple of the transmitted signals through the direct and multipaths, and then to select and combine some of them which have the better energy level among the received signals. Accordingly, the respective finger section separately processes an baseband signal transmitted from one transmitter and the same baseband signals received through another paths.
Thereafter, the early signal, late signal, and on-time signal are respectively inputted to three decimators in each finger section in the baseband signal demodulating apparatus at the point where on-time signal is synchronized with pseudo-noise code. The respective decimator selects one sample which is synchronized with pseudo-noise sequence among the oversampled signals for a predetermined period. Correlators are connected to respective decimeters one to one receiving early, late, and on-time samples, and correlation values between the output signals of the decimators and the pseudo-noise codes are respectively obtained. The correlation values are accumulated by accumulators connected to the correlators, respectively, for a predetermined period. The accumulated signals are transmitted to a symbol processor for further processing such maximum, deinterleaving, time tracking, decoding, and so on.
According to the conventional apparatus performing the above-described operations, correlators and accumulators, the number of which is the same as that of the decimators, should be provided to process multichannel signals received through the multipaths having the same contents, and the early, late, and on-time signals determined from the signal received through a path of a channel, respectively. Specifically, three correlators and three accumulators as well as a pseudo-noise code generator should be provided for each finger section, thereby complicating the construction of the whole baseband signal demodulating apparatus.
Accordingly, if the symbol processor has the same performance and the number of channels is continually increased due to the increase of the subscribers, the number of pseudo-noise code generators, correlators, and accumulators becomes increased as many as the number of channels. As a result, in order to increase the capacity of the conventional CDMA baseband demodulating apparatus, the hardware size of the base station system and the system construction cost should be greatly increased.
Accordingly, the present invention is directed to a baseband signal demodulating apparatus and method in a mobile radio communication system that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a baseband signal demodulating apparatus and method in a mobile radio communication system which can minimize the hardware size of the system due to the increase of channels by minimizing the number of correlators, accumulators, and pseudo-noise code generators and by sharing them by a time division multiplexing method.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the baseband signal demodulating apparatus in a mobile radio communication system comprises a plurality of front end rake receivers for demodulating oversampled baseband signals to restore the original signal, transmitting means for sharing demodulated signals from the front end rake receivers by a time division multiplexing method, and despreding means for despreding output signals of the transmitting means.
In another aspect of the present invention, there is provided a baseband signal demodulating method in a mobile radio communication system comprising the steps of demodulating oversampled baseband signals to restore the original signal, transmitting demodulated signals by a time division multiplexing method, and obtaining correlation values of transmitted signals by despreding the transmitted signals.
It is to be understood that both the foregoing general description and the following detailed description are explanatory and are intended to provide further explanation of the invention as claimed.